Ammonia has to be removed from gas streams that are vented into the air, because it causes environmental problems. Governmental regulations with respect to ammonia emissions will become stricter in future.
A process for the removal of ammonia from an ammonia containing gas stream is described, for instance, in U.S. Pat. No. 4,424,072. In this patent specification is described that the ammonia is removed from the gas stream exiting from the top of a prilling tower in a urea plant by contacting the gas stream in a scrubber with a non-volatile, dilute acid solution whereby the NH3 is absorbed. According to U.S. Pat. No. 4,424,072, non-volatile acid solutions include inorganic acids such as phosphoric acid, sulphuric acid and nitric acid as well as organic acids such as citric acid, oxalic acid and comparable non-volatile organic acids. According to U.S. Pat. No. 4,424,072 the ammonia-free gas stream is vented to the air. It is mentioned that the obtained aqueous stream comprising the ammonium salt leaving the scrubbing section in the prilling tower can be recycled to the urea production process.
The drawback of using the process according to U.S. Pat. No. 4,424,072, wherein the aqueous stream comprising the ammonium salt is recycled to the urea production process, is that contaminations by ammonium salts may occur in the final product, in this case urea, which contaminations are undesirable. For instance, ammonium salts in urea will usually render this urea unsuitable for the preparation of melamine. The separate processing of these ammonium salts (e.g. as a by-product) costs money and energy or often poses an environmental problem.
It has been found that the aforementioned drawback can be eliminated with a process wherein the aqueous stream comprising the ammonium salt is treated with electrodialysis, whereby the acid is recovered and an aqueous stream comprising an ammonium hydroxide salt is formed.
By performing the electrodialysis on the aqueous stream comprising the ammonium salt the acid that has been used to remove the ammonia can be recovered and an aqueous stream comprising the ammonium salt is not recycled to the urea production causing contamination of the produced urea.
According to the process for the removal of ammonia from an ammonia-containing gas stream according to the invention the aqueous stream comprising the ammonium salt is treated with electrodialysis.
Here and hereafter electrodialysis is defined as an electrolytical process comprising an anode and a cathode which also comprises at least one membrane situated between the anode and cathode. This membrane can be an anion-permeable membrane, a cation-permeable membrane or a combination of one or more of these membranes with at least two bipolar membranes. Anion-permeable membranes and cation-permeable membranes are permeable for anions respectively cations when these ions are attracted by the cathode respectively the anode. Bipolar membranes consist of an anion-permeable membrane and a cation-permeable membrane laminated together. When this bipolar membrane is oriented such that the cation-permeable membrane faces the cathode water is split into protons and hydroxyl ions.
During the treatment of the aqueous stream comprising the ammonium salt the acid used for converting the ammonia in the ammonia-containing gas stream is recovered. This acid can thus be reused for converting the ammonia in the ammonia-containing gas stream. Acids that can be used for converting the ammonia are, for example, the organic acids and inorganic acids mentioned in U.S. Pat. No. 4,424,072, as described above.
During the process according to the invention also an aqueous stream containing ammonium hydroxide is formed during the electrodialysis of the aqueous stream comprising the ammonium salt. After its formation this aqueous stream containing ammonium hydroxide can be heat treated, whereby a gaseous ammonia stream is formed.
The ammonia-containing gas stream can originate from various chemical processes, like the ammonia, urea and melamine production processes, but also from agricultural sources. The process according to the invention is in particular suitable for the treatment of ammonia-containing gas streams that contain low amounts of ammonia. These ammonia-containing gas streams are difficult to treat in an other way, for instance by separation and condensation, for removal of ammonia from these gas streams. Examples of ammonia-containing gas streams containing low amounts of ammonia are the gas streams that leave the prilling or the granulation sections of urea plants.
When the ammonia-containing gas stream, to be treated by the process according to the invention, is originating from the ammonia, urea or the melamine production processes the gaseous ammonia stream can be recycled to these processes. Before recycling the gaseous ammonia stream the gaseous stream can be treated to concentrate the ammonia in the gaseous stream.
Electrodialysis is performed in an electrochemical cell comprising an anode and cathode separated by an anion-permeable membrane or a cation-permeable membrane. Also a combination of one or more of these membranes with at least two bipolar membranes can be used. For the conversion of the ammonium salt in the aqueous stream in such a way that an acid is recovered and an ammonium hydroxide salt is formed, preferably the anode and the cathode are separated by at least one anion-permeable membrane. More preferably the electrochemical cell also comprises bipolar membranes and a cation-permeable membrane.
The invention is also directed to an electrodialysis section for treatment of an ammonia-containing gas stream comprising                a scrubber, wherein an ammonia-containing gas stream is contacted with an acid,        an electrodialysis apparatus, wherein a stream comprising an acid and a stream comprising an ammonium hydroxide salt are generated.Preferably, the electrodialysis section also comprises a stripper wherein the stream comprising an ammonium hydroxide salt is heated and a gaseous ammonia stream is formed.        
The electrodialysis section preferably is incorporated into a urea plant, comprising a prilling section or a granulation section, to treat the ammonia-containing gas stream leaving the prilling or granulation section.